The invention aims at improving the quality of the captured images when the image formation and detection conditions generate spectral aliasing, or when the signal-to-noise ratio of the image is insufficient.
A characteristic of an image capture instrument is its modulation transfer function, or MTF. This function characterizes the ability of the instrument to restore, in the captured images, patterns of the image-captured scene which have variable dimensions. Thus, the modulation transfer function is the contrast ratio between the image and the scene, for a pattern having intensity which varies sinusoidally and which is identified by its spatial frequency in the image. The modulation transfer function is obtained by varying the spatial frequency of the pattern, and by determining its contrast in the image as a function of this spatial frequency.
In a known manner, the capture of an image by the instrument has a low-pass filter effect. This effect appears as a decrease in the modulation transfer function as the spatial frequency of the pattern in the image increases.
Several contributions together constitute the modulation transfer function in the form of a product of these contributions, in order to constitute the modulation transfer function of the complete instrument. A first contribution is produced by the image formation optics which are used in the instrument. This first contribution is a decreasing function of the spatial frequency in the image, and is substantially zero for spatial frequencies which are greater than a cut-off frequency fC. This cut-off frequency is given by the following formula: fC=1/(N·λ), where:                λ is a wavelength of the radiation which is used in order to form the image, and which originates from the scene imaged by the instrument, and        N is the numerical aperture of the image formation optics: N=f/D, where f and D are, respectively, a focal length and a diameter of the pupil of these optics.        
Thus, the spectral components of the scene which would correspond to spatial frequencies in the image which are greater than the cut-off frequency fC, are removed from the image by the optics themselves. In other words, the image of the scene which is formed by the optics contains only spectral components the spatial frequencies of which are less than the cut-off frequency fC.
A second contribution to the modulation transfer function is produced by the image sensor which is used in the instrument. This sensor is situated in the focal plane of the image formation optics, and comprises at least one row of photodetectors which have identical individual dimensions and are aligned with a pitch p which is constant along the row.
In a known manner, such a sensor produces a sampling of the image, with a sampling frequency fE which is equal to 1/p, when an accumulation signal is read separately in each photodetector in order to constitute a different pixel of the captured image.
When the frequency of sampling fE by the photodetectors is less than twice the cut-off frequency fC, spectral aliasing occurs because of the level of sampling, which is insufficient. This spectral aliasing relates to the spectral components of the scene having spatial frequencies in the image which are comprised between fE/2 and fC. The intensities of these spectral components appear added to those the spatial frequencies of which are less than fE. The limit constituted by half of the sampling frequency fE vis-à-vis the ability of the sensor to provide a faithful representation of the spectral components of the image, is called the Nyquist frequency.
When the image sensor is assumed to be perfect, the value of its contribution to the modulation transfer function, for the spatial frequency in the image which is equal to half of the sampling frequency fE, is equal to sinc(π/2)≈0.64, when each photodetector separately produces an accumulation signal which corresponds to a different image pixel. In the preceding formula, sinc(x) is the sinus-cardinal function, or [sin(x)]/x when it is applied to an argument x.
The modulation transfer function which results from the image formation optics and the image sensor then has a value which is still significant for half of the sampling frequency fE. The spectral aliasing then produces artifacts in the captured image, which are visible in particular when this image is merged with another of the same scene but with a sampling frequency which is greater.
A solution for reducing these artifacts consists in reducing the size of the photodetectors in order to reduce their pitch p in the row of the sensor. In this way, the sampling of the image is increased. But the image sensor, with photodetectors which are smaller, is more complex. The data rates to be processed are then greater, which also leads to increased complexity for the data processing chain which is on-board. Moreover, the image sensor which is used can be required because of various constraints, and in particular by other uses of the same sensor when a multi-purpose mission is envisaged. In other words, the individual dimension and the pitch of the photodetectors may be required.
Another solution for reducing the artifacts due to the spectral aliasing consists in reducing the dimension of the pupil of the image formation optics. Thus, the value of the modulation transfer function at half of the sampling frequency is reduced. In this way, the parasitic contribution of the aliased frequencies in the contrasts of the spectral components of the image which have spatial frequencies of less than the sampling frequency, is decreased. However, the image formation optics can be common to several functions. In particular, these optics can be common to several image capture paths which are used simultaneously, for example with intervals which are different for the wavelength of the radiation. The dimension of the pupil of the image formation optics is then fixed, without being possibly adapted separately as a function of the sensor of each imaging path.
Finally, it is also known to vary the effective dimension and the effective pitch of the photodetectors for images which are captured, by “binning” photodetectors which are adjacent along a row of the sensor. To this end, the photodetectors are grouped together in row segments, successive along the row and separate. By “adjacent photodetector binning” is meant a mode for reading the accumulation signals from the photodetectors in which the respective signals from photodetectors which belong to one and the same segment are added together. FIG. 3 illustrates such mode for reading the image sensor, as known before the present invention. In this figure, the reference number 10 denotes the individual photodetectors of the image sensor 1, and S1′, S2′, S3′, S4′ etc. denote successive and separate grouping pairs of the photodetectors 10. Consequences of such binning of the photodetectors are in particular:                the spatial sampling frequency of the image is divided by two, which represents a reduction in imaging performance; and        the signal-to-noise ratio relative to each intensity value which is used in the image as it is captured, is improved.        
Such binning of the photodetectors, which aims at increasing the signal-to-noise ratio by accepting a degradation in the resolution, also gives rise to or worsens the image artifacts which are caused by the spectral aliasing.